The present invention relates generally to stylii for reproducing information signals recorded on information signal recording mediums. More particularly, the invention relates to a stylus for reproducing, as capacitance variation, an information signal recorded with high density as a variation in geometrical shape on an information signal recording medium of disc shape.
As one of systems, in which information signals such as video signals are recorded with high density on recording mediums of disc shape and are reproduced, there is a system which has been developed and reduced to practice, in its recording system, pits are formed along a track in accordance with an information signal in a recording medium substrate structure, and an electroconductive layer is adhered onto this substrate structure thereby to form a recording medium on which recording of the information has been completed. Then, in the reproducing system, this recording medium is rotated, and a reproducing stylus having an electrode is caused to trace relatively over and along the track of the medium. As a result, the elestrostatic capacitance between the electrode of the reproducing stylus and the electroconductive layer of the recording medium varies in accordance with the pits, whereby the information signal is reproduced in response to the variation of the capacitance.
Among the recording and reproducing systems of this so-called capacitance conversion type, there has been one in which, in its recording system, the pits are formed in accordance with the information signal on the bottom of a spiral groove in the surface of the recording medium, and, in the reproducing system, the reproducing stylus traces the track in a state wherein its stylus tip is within the spiral groove thereby to reproduce the information signal as variation of capacitance.
This known system, however, has had a drawback in that special reproducing modes of operation such as the so-called still reproduction, wherein a stopped picture is obtained by reproducing the recorded information of one revolution of the recording medium by repetition thereof through a plurality of revolutions, cannot be carried out. The reason for this is that, since the reproducing stylus is placed within the spiral groove in the recording medium, it would be forced to ride over the groove wall if an attempt were to be made to carry out this still reproduction. This would result in skipping of the stylus and also damage to the stylus and the groove wall of the recording medium. This case is true also in the slow motion picture and quick motion picture reproductions.
Another problem which has arisen in the prior art is due to the necessity of recording a video signal with high density on the recording medium since the information frequency band of a video signal, in general, is as large as 200 times that of an audio signal. For this reason, the width of the groove in the recording medium described above is made very narrow, being less than a number of .mu.m. Moreover, since the reproducing stylus is continually in sliding contact with only one groove, the area of the stylus tip in contact with the groove is small, whereby the contact pressure acting on the stylus tip per unit area thereof is extremely great such as 10 mg/.mu.m.sup.2, for example. For this reason, the wear of the reproducing stylus and the recording medium is remarkably rapid and gives rise to drawbacks such as short serviceable life.
If the stylus contact pressure is decreased with the aim of reducing this wear, so-called stylus skipping will occur, and stable reproduction cannot be achieved. Furthermore, a decrease in the stylus contact force will give rise to problems such as an increase in the effect of dust and other foreign matter adhering to the recording medium.
Accordingly, we have previously proposed a system wherein, in the recording system thereof, pits are formed in accordance with the information signal being recorded along a spiral track on a recording medium of flat disc shape, without forming a groove therein, and, in the reproducing system, a reproducing stylus traces over and along this track thereby to reproduce the recorded information signal. This system is disclosed in the specification of our U.S. patent application Ser. No. 785,095 filed Apr. 6, 1977, entitled "Information Signal Recording and Reproducing System". In this proposed system, pilot signals are recorded on or in the vicinity of a track of the information signal such as video signal on a rotary disc. At the time of reproducing, the pilot signals are reproduced together with the video signal, and tracking servo control is carried out so that the reproducing stylus traces accurately along the track in response to the reproduced pilot signals.
By the use of this previously proposed system, since the recording track has no groove, there is no possibility whatsoever of the reproducing stylus or the recording medium being damaged, and the stylus can trace the same portion of the track repeatedly a plurality of times, whereby a special reproduction such as still, slow motion, or quick motion reproduction becomes possible. Furthermore, while it is necessary for the electrode of the reproducing stylus to trace along only one track, a sliding contact surface of the main structure of the reproducing stylus for contacting with the recording medium can be made flat and with a wide width without being restricted by the width dimension of the recorded track, since the recorded track has no groove. For this reason, the rates of wear of the reproducing stylus and the recording medium are low, and the serviceable lives thereof are prolonged.
A conventional reproducing stylus used in systems such as the above described system previously proposed has the shape of a truncated triangular pyramid, an electrode being provided on one of its faces. For this reason, as this known reproducing stylus becomes worn with use, the width of the part thereof with the electrode becomes progressively large in proportion to the wear. Then, when this width of the electrode part becomes greater than the track pitch interval between adjacent track of the recording medium, the electrode part of the stylus begins to trace and reproduce simultaneously two adjacent tracks. As a consequence, beats occur in the reproduced signal, and the stylus becomes worthless for practical use. Accordingly, when the width of the electrode part becomes close to the track pitch interval, the life of the reproducing stylus ends. Therefore, while this conventional reproducing stylus has a longer life than the reproducing stylus of the system using a recording medium with a track groove, it is still accompanied by the problem of short life, which is of the order of a number of tens or hours, for example, in the case of a stylus having a main structure of sapphire.